Method of surface treating objects of cast magnesium base alloys



I Patented June 30,

METHOD OF SURFACE TREATING OBJECTS SIUM BASE ALLOYS OF CAST MAGNE GustavSiebel and Bitter! Eduard Naehtigau; cld, Germany No Drawing.Application January as, 1940, Serial No. 315,183. In Germany February23, 1939 liClaim. (01. 148-6.)

This invention relates to a method of surface treating objects of castmagnesium basealloys. For the production of objects, such as casings,fittings and the like, for example mounts for fleldglasses, castmagnesium base alloys are being used to an increasing extent, especiallyin the formof pressure die-castings. For such pressure clie-castings,magnesium base alloys are generally used which .cont aimin addition toother-alloying constituents, at least 4% of aluminium.

In order to impart to the surfaceof the metal a uniform appearance andat the same. time adequate resistance to corrosion, it is customary tosubject the surface of the cast objects to dipping or pickling treatmentwith known agents, such as dichromate solutions, It has however hithertobeen impossible, with the known dips, to obtain a layer of uniform,flawless appearance on the and an organic colouring matter notdecomposed by the said dichromate.

The heat treatment preceding the pickling tends to cause variations inthe crystalline texture of the'casting-bccasioned for example bysegregatiomnon-uniform distribution-of the constituents of the alloy andthe like-to disappear, this enabling a uniform and evenly colouredprotective layer to be produced by the dip or pickling liquid on theentire surface of the metals The heating of the present invention neednot however be carried so far that the heterogeneous structure of thealloy is converted into a completely homogeneous one by the formation ofa solid solution.

Example 1.-A housing of a field-glass, consist ing of a die-casting(magnesium base alloy contaming 7.5% of aluminium), is heated to atemsurface of the'casting;- moreover, it was not possible to obtainuniform surface layers of dark to black {coloration on pressure.die-cast objects consisting of magnesium base alloys having an aluminiumcontent of over 4%. "Since the layers are also intended to afford a moreor less effective protection against corrosion to the base metal, it isparticularly important that the surface layer should be uniform.

According to the present invention, objects of the foregoing kind aresubjected to a combined treatment which consists in first subjecting thecastings to heating at a temperature above' 320 C. for several hours,whereupon, after the cooling which need not necessarily be effected byquenching, a protective layer is produced on the thus pretreated castingby one of the known clipping or pickling processes.

The present invention advantageously maybe I used as a preliminarytreatment to a p rocess for the-production of coloured protectivelayerson castings of magnesium base alloys as described in a co-pendingapplication Ser. No.- 315,184, entitled Process for the production ofcoloured protective layers on castings, especially die-castings, ofmagnesium base alloys and filed on even date' herewith, of whichapplication we are two of the joint inventors. The said applicationrelates to a process for the production of coloured protective layers onmagnesium base alloy castings, especially pressure die-castings,suchalloys containing more than 4% of aluminium, which comprisesfirstsubjecting the casting to a prechromate, and thereupon treating thecasting in a dip or pickling solution containing a dichromate i.liminary pickling in a bath containing a diperature of 360 C.forthreehours and thereupon cooled in the air to room-temperature. Afterbeing pickled in 8% nitric acid-an operation taking a. few seconds-andsubsequently being rinsed in water, the object is placed in an aqueousdip containing per litre 100 grams of a salt mixture composed of equalparts of sodium dichromate and magnesium sulphate, and. shortcircuitedoutside the bath against an electrode of a more electronegative metal,such as iron, also suspended in the solution. The housing is removedfrom the bath after about 30 minutes,

briefly rinsed in hot water and dried in theatmosphere. On the surfaceof the metal has been j deposited a protective layer of matt.appearance,

'which is uniformly black in colour and which imparts to the base metala high degree of protection against atmospheric corrosion.

7 Example 2.--A pressure die-casting consisting of a magnesium basealloy containing 9% of alu-' minium and 1% of zinc, is heated to atemperature of 350 C. for three hours and thereupon cooled in the air toroom temperature. After being pickled in nitric acid of about 8%strength '--an operation taking a few seconds-and subsequently rinsed inwater, the object-istreated with an aqueous solutioncontaining 100grams.

of aimi xture of'lO grams of magnesium sulphate and .ofldichromate perlitre at a temperat'ure of 90 C. After 30 minutes the object is removedfrom the solution and, without'previous rinsing, is dipped in acolouring bath heated to a temperature of 90 C. and containing 50 gramsof sodium dichromate and 20 grams of 'water-soluble nigrosin (seeFarbstofftabellen Gustav Schultz, .6th edition, 1931, No. 986) per litreof aqueous solution, and is left therein for 15 minutes. After removalfrom the bath, the casting is washed for a short time in hot water andair-dried. The deep black, tightly adhering protective layer depositedon the metal has a matt appearance and protects the surface of thecasting in a high degree against corrosion.

We claim:

A process for producing a coating of uniform appearance upon a castingof a magnesium base alloy containing at least about 4 per cent ofaluminum, which comprises subjecting said magnesium base alloy castingto a heat treatment 1 at a temperature above about 320 of timesuflicient to cause at least genization of the alloy, and thensubjecting said casting to a preliminary pickling in a bath containingadichromate and being tree from organic coloring matter and thereaftertreating said casting in a solution containing a dichromate and anorganic coloring agent not decomposed bythe said dichromate.

C. for a period GUSTAV, SIEBEL. EDUARD NACH'I'IGALL.

partial homo-

